Method for oligomerizing C6-olefins

ABSTRACT

In a process for oligomerizing C6-olefins by reactin of a C6-olefin-containing reaction mixture over a nickel-containing fixed-bed catalyst, the reaction over the fixed-bed catalyst is run at a conversion to oligomerized C6-olefins of not more than 30% by weight, based on the reaction mixture.

The present invention relates to a process for oligomerizing C₆-olefins,in particular for preparing C₁₂-olefins by dimerization.

Processes for the oligomerization of olefins are known. DE-A-43 39 713describes a process for oligomerizing olefins to give highly linearoligomers. In this process, C₂₋₆-olefins are reacted at superatmosphericpressure and elevated temperature over a fixed-bed catalyst comprisingfrom 10 to 70% by weight of nickel oxide, from 5 to 30% by weight oftitanium dioxide and/or zirconium dioxide, from 0 to 20% by weight ofaluminum oxide as significant active constituents and silicon dioxide asthe remainder.

U.S. Pat. No. 4,959,491 describes a process for dimerizing C₆-olefins toform C₁₂-olefins which can be used for preparing surfactants. Catalystused are nickel-containing catalysts such ashexafluoro-acetoacetylnickel(cyclooctadiene).

DE-A-39 14 817 describes a process for oligomerizing C₂₋₈-olefins, inwhich the reaction is carried out over nickel-exchanged montmorillonite,a nickel-aluminum-silicon oxide catalyst or nickel-impregnated molecularsieves or zeolites. The olefin mixture used is passed over a molecularsieve prior to the catalytic reaction.

A disadvantage of the known processes is that the catalyst life isfrequently too short. The catalyst is, in particular, clogged by higheroligomers and therefore loses its activity.

It is an object of the present invention to provide a process foroligomerizing C₆-olefins which avoids the disadvantages of the knownprocesses.

We have found that this object is achieved by a process foroligomerizing C₆-olefins by reaction of a C₆-olefin-containing reactionmixture over a nickel-containing fixed-bed catalyst, wherein thereaction over the fixed-bed catalyst is run at a conversion tooligomerized C₆-olefins of not more than 30% by weight, based on thereaction mixture.

The reaction over the fixed-bed catalyst is preferably carried out at aconversion of from 10 to 30% by weight, particularly preferably from 10to 25% by weight, based on the reaction mixture. The oligomerization ispreferably essentially a dimerization.

According to the present invention, it has been found that deactivationof the catalyst can be avoided and the dimer selectivity can beincreased if the conversion over the catalyst is in the range indicated.The process can be carried out batchwise or continuously. It ispreferably carried out continuously in the liquid phase. The conversionis then based on a throughput of the reaction mixture through thecatalyst.

The reaction is preferably carried out at from 30 to 300° C. and apressure in the range from 10 to 300 bar.

In order to achieve a high total conversion in the process, part of theunreacted reaction mixture obtained can, after separating off theoligomers, be returned to the reaction. Adjustment of the recycledamount of reaction mixture enables very high total conversions to beachieved. The term “oligomers” includes dimers and higher-boilingcompounds.

The process of the present invention makes it possible to realize atotal conversion of over 90% together with a C₁₂ selectivity of over80%. Adherence to the conversion specified according to the presentinvention over the catalyst (based on a single pass) greatly increasesthe operating life of the catalyst, since the formation of high-boilingcompounds which can deposit on the catalyst and thus cause a drop inactivity is suppressed.

C₆-olefins which are suitable for use in the process of the presentinvention can be synthesized on an industrial scale by methods such aspropylene dimerization. The most important industrial propylenedimerization processes are described, for example, in A. Chauvel and G.Lefebvre, Petrochemical Process, Edition Technip (1989), pp. 183 to 187and F. Asinger, Die petrochemische Industrie, Akademier-Verlag (1971),pp. 278 to 299. The oligomerization is carried out industrially in thepresence of either homogeneous or heterogeneous catalysts. Theheterogeneous catalysts which can be used are listed in, for example,C.T. O'Connor et al., Catalysis Today Vol. 6 (1990), pp. 329 to 349.

The most important, based on the amount produced, homogeneouslycatalyzed process is the Dimnerol-G process of IFP. It is described indetail in Erdöl, Erdgas and Kohle, number 7/8, July/August 1990, pp. 309to 315. The product obtained by means of this process (known as“Dimate”) has the following average olefin

C₃: 4% by weight C₆: 73% by weight C₉: 17% by weight C₁₂: 4% by weightC₁₅₊: 2% by weight

The C₆ fraction is composed of:

4-methyl-1-pentene: 0.9% by weight 2,3-dimethyl-1-butene: 2.3% by weightcis-4-methyl-2-pentene: 3.1% by weight trans-4-methyl-2-pentene: 21.7%by weight 2-methyl-1-pentene: 5.0% by weight 1-hexene: 0.3% by weighttrans-3-hexene: 4.4% by weight cis-3-hexene: 0.7% by weighttrans-2-hexene: 13.6% by weight 2-methyl-2-pentene: 39.2% by weightcis-2-hexene: 3.7% by weight 2,3-dimethyl-2-butene: 4.8% by weight

Another source of C₆-olefins is provided by metathesis processes.

Possible catalysts are generally nickel-containing catalysts known perse which give little branching, as are described, for example, inCatalysis Today vol. 6 (1990), pp. 336 to 338, DE-A43 39 713, U.S. Pat.No. 5,169,824, DD2 73 055, DE-A-20 51 402, EP-A-0 202 670, Appl. Catal.31 (1987), pages 259 -266, EP-A-0 261 730, NL 8 500 459, DE-A-23 47 235,U.S. Pat. No. 5,134,242, EP-A-0 329 305, U.S. Pat. Nos. 5,146,030,5,073,658, 5,113,034 and 5,169,824.

In a preferred embodiment of the process of the present invention, theoligomerization is carried out in the liquid phase using the catalystsdescribed in DE-A 43 39 713.

The catalysts described there consist essentially of nickel oxide,silicon oxide, titanium oxide and/or zirconium oxide and, if desired,aluminum oxide and have a nickel oxide content of from 10 to 70% byweight, a content of titanium dioxide and/or zirconium dioxide of from 5to 30% by weight and an aluminum oxide content of from 0 to 20% byweight, with the remainder being silicon dioxide. They are obtainable byprecipitation of the catalyst composition at a pH of from 5 to 9 byaddition of an aqueous solution of nickel nitrate to an alkali metalwater glass solution containing titanium oxide and/or zirconium dioxide,filtration, drying and heating at from 350 to 650° C.

The catalysts preferably contain essentially from 10 to 20% by weight oftitanium dioxide, from 0 to 10% by weight of aluminum oxide and from 40to 60% by weight of nickel oxide as main constituent and activecomponent and silicon dioxide as the remainder.

Especially preferred catalysts have the composition 50% by weight ofNiO, 34% by weight of SiO₂, 3% by weight of Al₂O₃ and 13% by weight ofTiO₂. They are largely free of alkali metals (Na₂O content <0.3% byweight).

The catalysts are preferably arranged in a fixed bed and are thereforepreferably in the form of discrete bodies, e.g. in the form of pellets(5 mm ×5 mm, 5 mm ×3 mm, 3 mm ×3 mm), rings (7 mm ×7 mm ×3 mm, 5 mm ×5mm ×2 mm, 5 mm ×2 mm ×2 mm) or extrudates (1.5 mm diameter, 3 mmdiameter, 5 mm diameter).

In the process of the present invention, preference is given to reactinga hydrocarbon stream comprising n-hexene and/or methylpentene,preferably in the liquid phase, over the abovementioned Ni-containingcatalysts.

Suitable C₆-hydrocarbons are, for example, mixtures having the followingcomposition:

paraffin: from 10 to 90% by weight olefin: from 10 to 90% by weight,

where the olefin fraction can have the following composition:

n-hexenes: preferably from 0.1 to 99.8% by weight methylpentenes:preferably from 0.1 to 99.8% by weight dimethylbutenes: preferably from0.1 to 99.8% by weight

The hydrocarbon streams used are advantageously freed ofoxygen-containing compounds such as alcohols, aldehydes, ketones orethers by adsorption using a protective bed such as molecular sieves,aluminum oxides, aluminum oxide-containing solids, aluminum phosphates,silicon dioxides, kieselguhr, titanium dioxides, zirconium dioxides,phosphates, carbonontaining adsorbents, polymer adsorbents or mixturesthereof, as is known per se from DEA 39 14 817.

The oligomerization reaction takes place at from 30 to 300° C.,preferably from 80 to 250° C. and in particular from 100 to 200° C., anda pressure of from 10 to 300 bar, preferably from 15 to 100 bar and inparticular from 20 to 70 bar. The pressure is advantageously chosen sothat the feed mixture is in liquid form at the temperature set. Thereactor is generally a cylindrical reactor or shaft oven charged withthe catalyst and the liquid reaction mixture flows through it from thetop downward. After leaving the single-stage or multistage reactionzone, the oligomers formed are separated from the unreactedC₆-hydrocarbons in a manner known per se (e.g. by distillation) and allor most of the latter is returned to the reaction (however, a certainpurge to remove inerts, e.g. hexane, is always necessary).

A useful aspect of the method of carrying out the reaction provided bythe present invention is the opportunity of carrying out the processadiabatically in a shaft oven, since the heat generated in the reactorcan be controlled as desired by dilution of the hexenes with therecirculated stream by choosing the amount and temperature of thisstream. Compared to an isothermally operated process, the adiabaticprocedure leads to a considerable reduction in the capital costs of theapparatus.

In one embodiment of the invention, it is possible to fractionate thefeed mixture in a column (K) to separate C₆-olefins and oligomers(C₇₊-hydrocarbons) prior to the reaction, to pass the C₆-olefins to thereaction (C1), to return the reacted mixture to the column (K1) and todischarge the oligomers (C₇₊-hydrocarbons).

In a further embodiment, it is possible to fractionate the reactedmixture after the reaction in a column (K1) to separate C₆-olefins andoligomers, to return the C₆-olefins to the reaction (C1) and todischarge the oligomers.

The two abovementioned variants are shown schematically in FIG. 1a) andb) in the accompanying drawing.

In the figures, the symbols have the following meanings:

F1: protective bed C1: reactor K1: column F: feed P: purge D: distillateS: bottoms

The protective bed (F1) serves to remove catalyst poisons (essentiallyS—N—O—containing hydrocarbons).

The fractionation of the oligomers is carried out in a manner known perse by fractional distillation to separate off the desired dodecenes. Thesulfur-free C₁₃₊fraction displays a high blend value in respect ofmixing into the diesel fuel pool. This C₁₃₊ fraction is particularlypreferably used as diesel fuel component after the olefins have beenconverted into paraffins by hydrogenation. This measure increases thecetane number which is a critical measure of the properties of thediesel fuel. All methods known from the prior art can be used for thehydrogenation.

The dodecenes obtained from the hexene dimerization can be furtherprocessed to produce surfactants.

The following examples illustrate the process of the present invention.

EXAMPLES

The experimental plant comprises the following plant items (processdiagram as in FIG. 1):

adsorber for removing catalyst poisons (F1, volume: about 50 l)

adiabatic reactor (C1, volume: about 40 l, length: 8 m, diameter 80 mm)

distillation column (K1) for separating unreacted C₆-olefins and theoligomers formed [C₁₂].

The catalyst used was a material which had been produced in the form of5 mm×5 mm pellets as described in DE-A 43 39 713. Composition in % byweight of the active components: 50% by weight of NiO, 13% by weight ofTiO₂, 34% by weight of SiO₂, 3% by weight of Al₂O₃.

As adsorbent, use was made of a high surface area aluminum oxide such asSelexsorbo ® from Alcoa.

Example 1

The feed mixture used was a hydrocarbon mixture having the followingcomposition:

C₃: 4% by weight C₆: 73% by weight C₉: 17% by weight C₁₂: 4% by weightC₁₅₊: 2% by weight

The C₆ fraction is composed of:

4-methyl-1-pentene: 0.9% by weight 2,3-dimethyl-1-butene: 2.3% by weightcis-4-methyl-2-pentene: 3.1% by weight trans-4-methyl-2-pentene: 21.7%by weight 2-methyl-1-pentene: 5.0% by weight 1-hexene: 0.3% by weighttrans-3-hexene: 4.4% by weight cis-3-hexene: 0.7% by weighttrans-2-hexene: 13.6% by weight 2-methyl-2-pentene: 39.2% by weightcis-2-hexene: 3.7% by weight 2,3-dimethyl-2-butene: 4.8% by weight.

The hydrocarbon mixture was introduced into the column K1 (FIG. 1A) at arate of 5.1 kg/h. The following conditions were set in the experimentalplant:

Adsorption section: Pressure (bar) 15 Temperature (° C.) 35 Throughput(kg/h) 18.8 Synthesis section: Amount of catalyst (kg) 25 Pressure (bar)15 Inlet temperature (° C.) 100 Outlet temperature (° C.) 139 Throughput(kg/h) 18.8 Distillation section: Pressure (bar) 1 Temperature - top (°C.) 35 Temperature - bottom (° C.) 185 Amount fed in (kg/h) 23.9Distillate (kg/h) 19.0 Purge (kg/h) 0.2 Bottoms (kg/h) 4.9

The following result was achieved:

Stream C₃ C₆ C₉ C₁₂ C₁₅₊ Total C₉₊ Feed mixture to K1 =   1.7 78.1 3.713.4 3.1 20.2 reactor output Distillate from K1   2.1 97.9 <0.1 <0.1<0.1 — Bottoms from K1 <0.1  0.4 17.7 64.7 17.2 99.6

This gives a C₆-olefin conversion of 94.7% and a C₁₂ selectivity of83.6% (based on the C₆-olefins reacted).

Example 2

The feed mixture used was a hydrocarbon mixture having the followingcomposition:

C₅: 0.9% by weight C₆: 98.7% by weight C₇: 1.2% by weight

The C₆ fraction is composed of:

4-methyl-1-pentene: <0.1% by weight 2,3-dimethyl-1-butene: <0.1% byweight cis-4-methyl-2-pentene: <0.1% by weight trans-4-methyl-2-pentene:<0.1% by weight 2-methyl-1-pentene: <0.1% by weight 1-hexene: <0.1% byweight trans-3-hexene: 90% by weight cis-3-hexene: 10% by weighttrans-2-hexene: <0.1% by weight cis-2-hexene: <0.1% by weight2-methyl-2-pentene: <0.1% by weight 2,3-dimethyl-2-butene: <0.1% byweight.

The hydrocarbon mixture was introduced into the filter F1 (FIG. 1B) at arate of 3.20 kg/h. The following conditions were set in the experimentalplant:

Adsorption section: Pressure (bar) 10 Temperature (° C.) 35 Throughput(kg/h) 3.20 Synthesis section: Amount of catalyst (kg) 25 Pressure (bar)10 Inlet temperature (° C.) 100 Outlet temperature (° C.) 133 Throughput(kg/h) 15.75 Distillation section: Pressure (bar) 1 Temperature - top (°C.) 45 Temperature - bottom (° C.) 182 Amount fed in (kg/h) 15.75Distillate (kg/h) 12.60 Purge (kg/h) 0.05 Bottoms (kg/h) 3.15

The following result was achieved:

Stream C₅ C₆ C₇₋₁₁ C₁₂ C₁₃₊ Total C₇₊ Feed mixture to K1 = <0.1 80.6  0.4 15.7 3.3 19.4 reactor output Distillate from K1   0.1 99.9 <0.1<0.1 <0.1 — Bottoms from K1 <0.1  0.4   1.3 81.2 17.1 99.6

This gives a C₆-olefin conversion of 98.4% and a C₁₂ selectivity of82.6% (based on the C₆-olefins reacted).

We claim:
 1. A process for oligomerizing C₆-olefins by reaction of aC₆-olefin-containing reaction mixture over a nickel-containing fixed-bedcatalyst, comprising from 10 to 70% by weight of nickel oxide, from 5 to30% by weight of titanium dioxide and/or zirconium dioxide and from 0 to20% by weight of aluminum oxide as significant active constituents andsilicon dioxide as the remainder, wherein the reaction over thefixed-bed catalyst is carried out continuously in the liquid phase andrun at a conversion to oligomerized C₆-olefins of from 10 to 30% byweight based on a throughput of a reaction mixture through the catalystin a single pass.
 2. A process as claimed in claim 1, wherein thereaction over the fixed-bed catalyst is run at a conversion tooligomerized C₆-olefins of from 10 to 25% by weight, based on thereaction mixture.
 3. A process as claimed in claim 1, wherein theoligomerization is essentially a dimerization.
 4. A process as claimedin claim 1 carried out at from 30 to 300° C. and a pressure in the rangefrom 10 to 300 bar.
 5. A process as claimed in claim 1 which is carriedout adiabatically in a shaft oven and in which part of the reactionmixture is returned to the reaction.
 6. A process as claimed in claim 1,wherein a feed mixture is fractionated in a column to separateC₆-olefins and oligomers prior to the reaction, the C₆-olefins arereturned to the reaction, the reaction mixture is returned to the columnand the oligomers (C₆-hydrocarbons) are discharged.
 7. A process asclaimed in claim 1, wherein the reaction mixture after the reaction isfractionated in a column to seperate C₆-olefins and oligomers, theC₆-olefins are returned to the reaction and the oligomers aredischarged.
 8. A process as claimed in claim 1, wherein the reactionmixture is passed over a protective bed wherein catalyst poisons areremoved, prior to the reaction.